Make more arch-specific #if's exclusive with #else #error cases

Make more arch-specific #if's exclusive with #else #error cases
So when the next person compiles the Sparc NCG it should fail more
obviously at compile time rather than panicing at runtime.
Plus one obvious fix for LocalReg gaining an extra param
Missing bits of Sparc NCG:
  * genSwitch for generating jump tables. This is the most tricky one.
  * ALLOCATABLE_REGS_INTEGER and ALLOCATABLE_REGS_DOUBLE just requires
    finding and verifying the values. The nearby comment describes how.
  * isRegRegMove and mkRegRegMoveInstr. Sparc uses Or for int move, check
    what this is supposed to do for single and double float types.
  * regDotColor. Probably just copy the ppc impl.